The invention relates to measuring cell voltages of a fuel cell stack.
A fuel cell is an electrochemical device that converts chemical energy produced by a reaction directly into electrical energy. For example, one type of fuel cell includes a proton exchange membrane (PEM), a membrane that may permit only protons to pass between an anode and a cathode of the fuel cell. At the anode, diatomic hydrogen (a fuel) is oxidized to produce hydrogen protons that pass through the PEM. The electrons produced by this oxidation travel through circuitry that is external to the fuel cell to form an electrical current. At the cathode, oxygen is reduced and reacts with the hydrogen protons to form water. The anodic and cathodic reactions are described by the following equations:
H.sub.2.fwdarw.2H.sup.+ +2e.sup.- at the anode of the cell, and
O.sub.2 +4H.sup.+ +4e.sup.-.fwdarw.2H.sub.2 O at the cathode of the cell.
Because a single fuel cell typically produces a relatively small voltage (around 1 volt, for example), several fuel cells may be formed out of an arrangement called a fuel cell stack to produce a higher voltage. The fuel cell stack may include plates (graphite composite or metal plates, as examples) that are stacked one on top of the other, and each plate may be associated with more than one fuel cell of the stack. The plates may be made from a graphite composite material and include various channels and orifices to, as examples, route the reactants and products through the fuel cell stack. Several PEMs (each one being associated with a particular fuel cell) may be dispersed throughout the stack between the anodes and cathodes of the different fuel cells.
The health of a fuel cell stack may be determined by monitoring the individual different terminal voltages (herein called cell voltages) of the fuel cells. In this manner, a particular cell voltage may vary under load conditions and cell health over a range from -1 volt to +1 volt. The fuel cell stack typically may include a large number of fuel cells, and thus, common mode voltages (voltages with respect to a common voltage (ground)) of the terminals of the fuel cells 16 may be quite large (i.e., some of the voltages of the terminals may be near 100 volts, for example). Unfortunately, semiconductor devices that may be used to measure the cell voltages typically are incapable of receiving high common mode voltages (voltages over approximately 18 volts, for example).
For example, referring to FIG. 1, in a fuel cell system 10, a fuel cell voltage monitoring circuit 14 may be used to measure the differential voltages across fuel cells 16 (fuel cells 16.sub.1, 16.sub.2 . . . 16.sub.N, as examples) of a fuel cell stack 12. The stack 12 forms an overall stack voltage called V.sub.CELL. Because the fuel cells 16 are serially coupled together, the common mode voltage of a particular cell 16 becomes progressively greater the farther the cell 16 is away from the ground connection. For example, the cell voltages of the terminals 18 and 19 may have relatively low common mode voltages, as the voltages of the terminals 18 and 19 are formed from one fuel cell 16.sub.1 and two fuel cells 16.sub.1 and 16.sub.2, respectively. However, farther from the ground connection, a cell terminal 20 has a much higher common mode voltage.
Because the voltage monitoring circuit 14 may include a semiconductor device that cannot receive large common mode voltages (a voltage above approximately 18 volts, for example) at its input terminals, the voltage monitoring circuit 14 may include circuitry to scale down the voltage(s) that are furnished by the cell terminals. In this manner, the voltage monitoring circuit 14 may use the circuitry to indicate a scaled down version of a particular cell voltage and then derive an indication of the actual cell voltage by upscaling the scaled down value by the appropriate amount. For example, the circuitry may scale down the voltages by a factor of 10. Therefore, for this example, the circuitry may receive a voltage of 100 volts and provide a corresponding voltage of 10 volts to a semiconductor that is used to measure the cell voltage, for example.
As a more specific example, resistive dividers (not shown in FIG. 1) may be coupled inline with measurement terminals 15 (of the voltage monitoring circuit 14) that extend to different cell terminals of the fuel cell stack 12. If all of the resistive dividers reduce the common mode voltages by the same amount, then the system 10 works as desired. However, unfortunately, the resistive dividers may have tolerances due to the tolerances of their resistive components. These tolerances, in turn, may cause common mode voltages to be introduced into the measurements of the cell voltages.
For example, the voltage monitoring circuit 14 may attempt to measure a voltage V.sub.N (i.e., the cell voltage of the fuel cell 16.sub.N) by conducting a voltage measurement across cell terminals 20 and 21. However, because of the non-ideal circuitry, such as resistive dividers, that are used to scale down the measured voltage, the monitoring circuit 14 may effectively measure the following voltage (called V.sub.MEAS): EQU V.sub.MEAS =K.sub.1.multidot.V.sub.N +K.sub.2.multidot.V.sub.CM, (Equation 1)
where K.sub.1 is a differential gain (ideally the scaling factor that is supposed to be introduced by the resistive divider), K.sub.2 is a common mode gain (ideally zero) and V.sub.CM is the voltage common to the cell terminals 20 and 21. Thus, as can be seen from Equation 1, the V.sub.MEAS measured voltage may not be proportional to the cell voltage V.sub.N, but instead, the V.sub.MEAS measured voltage may include a common mode component, K.sub.2.multidot.V.sub.CM. This common mode component, in turn, introduces an error into the measurement and thus, may inhibit the ability of the voltage monitoring circuit 14 to accurately monitor the health and load of the cells 16. This inaccuracy, in turn, may impede the ability of the voltage monitoring circuit 14 to recognize when a particular fuel cell has failed or is about to fail and thus, may impede the ability of the voltage monitoring circuit 14 to take the appropriate corrective action (an action that includes shutting down the stack 10 or alerting an operator, as examples).